Permanent magnets can be used for establishing a magnetic field in electric machinery. In permanent magnet electric machines, permanent magnets are fitted in a rotor that is at an air gap distance from the electric machine's stator. The permanent magnets can be used for establishing a magnetic field, and the magnetic flux of the field goes over the air gap to the stator. The permanent magnets can be fitted either on the rotor's surface, or inside the rotor's magnetically conductive frame. The air gap between the rotor and the stator can be parallel to the electric machine's shaft, or perpendicular to the shaft, in which case the air gap can be radial. In electric machines with a radial air gap, the rotor can be either inside or outside the stator.
The disclosure is related to surface-fitted permanent magnets, and electric machines that have been magnetized with permanent magnets and which have a radial air gap. A permanent magnet can include one or more permanent magnet pieces in a direction of the electric machine's shaft. Each pole of the electric machine has one or more parallel permanent magnet pieces in the circumferential direction of the electric machine. The electric machine can have an external rotor or an internal rotor, and the permanent magnets can be fitted on a rotor surface that is facing towards the stator. The electric machine can function either as a generator or a motor. Moreover, the electric machine can have at least ten poles but the number of poles can be many dozens, up to over a hundred poles. The electric machine's number of slots per pole per phase can be one or two.
In electric machines, it is desirable to establish in the air gap a magnetic flux density that varies as evenly as possible in the electric machine's magnetic pole area. The magnetic flux density is the highest in the middle of the magnetic pole, decreases ideally according to a sinusoidal curve when moving towards the pole's edge, and is zero on the pole edge. If the air gap influx distribution deviates extensively from the pure sinusoidal form, harmonic waves in the distribution can cause torque vibration. In permanent magnet electric machines, in which the number of slots per pole per phase is one or two and which have a large number of poles, idling can create a cogging torque due to the permeance fluctuation caused by the stator tooth. Under load, the current flowing in the stator winding can result in a flux that causes torque ripple. The cogging torque and the torque ripple caused by the current are summed under load. The dimensioning guideline is that during idling, the cogging torque may not exceed 1% of the nominal torque. On the other hand, under nominal load, the torque ripple may not exceed 2% of the nominal torque.
Powerful permanent magnets such as NdFeBo [neodymiumironboron] magnets based on rare earth metals can be used in permanent magnet electric machines. They can provide a sufficient magnetic field, but can be fragile, and it is difficult, time consuming and expensive to process them exactly to the intended shape.
Fastening permanent magnets on the rotor's curved surface involves processing. Either the rotor surface can be processed straight piece by piece, or the lower surface of the permanent magnet can be processed concave. Besides gluing used for the fitting, permanent magnets may also have to be fastened with fastening means between the poles. The fastening means can take up space from the permanent magnet itself in the circumferential direction of the rotor.
Permanent magnet dimensions' impact on the performance of the electric machine can be a complicated problem where several factors affect the outcome, often in a conflicting manner. Therefore, an optimal outcome is the combined effect of many factors.
A permanent magnet electric machine is known from the published patent application JP 01-234038 where permanent magnets, their cross-section being the shape of a hexagon, have been fitted on the rotor surface. The rotor's outer circumference has been processed straight at the poles, in which case the cross-section of the rotor frame is a polygon. The permanent magnets are shaped so that there is a straight middle section on an upper surface of the permanent magnet. The middle section is small in order to achieve a minimal cogging torque.